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Help with some uncertainty questions? Watch

1. Q1: To say you measured using a meter rule a length of an object 3 times to get the following readings : 61 mm , 64 mm , 65 mm how do we find out the uncertainty?

Q2: To say you measured a diameter of a ball and the percentage uncertainty is 1 % what will be the the percentage uncertainty of its radius? (Do we divide by 2 because i remember in another question to find the perimeter of a rectangle we multiplied the uncertainty of the length and width by 2 to find the total uncertainty)
2. Uncertainty is the most accurate you can measure to/value you measured.
So a metre rule, you can accurately measure to half a mm. So 0.5/61 times 100. Well you'd find the average of those three values and then do 0.05/average x 100
3. (Original post by eggfriedrice)
Uncertainty is the most accurate you can measure to/value you measured.
So a metre rule, you can accurately measure to half a mm. So 0.5/61 times 100. Well you'd find the average of those three values and then do 0.05/average x 100
thanks
4. The %age uncertainty in the radius is the same.
The absolute uncertainty is half that for the diameter.
5. (Original post by Stonebridge)
The %age uncertainty in the radius is the same.
The absolute uncertainty is half that for the diameter.
i dont quite follow?
6. (Original post by >>MMM<<)
i dont quite follow?
If you measure the diameter as, say, 100 ± 2cm that is a % uncertainty of 2% and an absolute uncertainty of ± 2cm

You then calculate the radius as 50cm
The actual uncertainty is also halved giving radius = 50 ± 1cm
This is still a % uncertainty of 2%
1 in 50 is 2%
2 in 100 is 2%
The % uncertainty remains the same on division (or multiplication) by a pure number. This is because there is no uncertainty in the value (here) of 2 that you divide the diameter by.

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Updated: April 9, 2013
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